In electrical power systems, power generation and load demand must be balanced at all times. In the event load demand is higher than the current power generation, load shedding must be initiated in order to correct the imbalance and maintain system stability and operability. This balance is reflected in the frequency response of the power system, i.e. how the frequency of the generated power aligns with a target frequency reflecting a stable system, e.g. 60 Hz. For example, when load demand becomes greater than power generation, the frequency of the generated power will drop, unless and until either the power generation increase accordingly, or the excess load is removed.
Load shedding is the removal of excess load from a power system to keep the system stable and operational. Load shedding is typically in response to one or more system disturbances, also known as “triggers” or “contingencies,” that result in a power generation deficiency condition, i.e. a reduction in power generation. Common contingencies that can cause a deficiency condition include faults, loss of generation, switching errors, lightning strikes, and other similar events.
Sudden and large changes in power generation capacity, e.g. through the loss of a generator or main inter-tie, may impact the dynamic response of the prime mover, which can produce severe power generation and load imbalances, resulting in rapid frequency decline. For some disturbances, notably those resulting in a loss of generation or a system islanding effect, the cascading effects of these sudden and large power generation changes may be of primary concern. Indeed, if load shedding is not set and timed correctly these cascading effects may cause more of a risk to overall system stability than the initial event itself.
For example, a short circuit at a power station busbar may initially result in an acceleration of the generator prime movers (e.g. turbines) due to the decreased load sensed by the power system because of the fault, i.e. frequency rises above the target. When this occurs, a speed regulator may act to decelerate the prime movers in order to correct the imbalance, i.e. lower the frequency to the target. However, once the fault clears, the prime movers face are faced with the impact of the actually still connected load, i.e. frequency is actually below target, except now under difficult reacceleration conditions.
This drop in system frequency may instigate a rapid fall of power output to the auxiliary loads, causing further reductions of the energy input to the generator prime movers. This sequence of events may further deteriorate the system frequency, endangering the entire power system. To halt the drop in system frequency, it is necessary to intentionally disconnect a portion of the load equal to or greater than the generation deficiency in order to achieve balanced power economics while maintaining system stability. Automated load shedding systems are therefore necessary for industrial power systems since sudden disturbances can plunge a power system into a hazardous state much faster than an operator can react.
Conventional automated load shedding schemes utilize fixed frequency settings with fixed frequency relays. For example, an under-frequency rely load shedding scheme utilizes fixed load reduction at fixed system frequency levels. If the power system frequency falls below a frequency set point for a pre-specified period of time, the frequency relay trips one or more load breakers, i.e. sheds a pre-determined load or loads. This cycle is repeated until the power system frequency is recovered, e.g. 10% load reduction for every 0.5% frequency drop. However, in the time it takes for the frequency relay to trip the load breakers to shed the predetermined load(s) for that frequency level, the frequency may have degenerated past the level where such a response is sufficient, instigating successive load shedding operations in the face of a continuingly degenerative frequency level, and risking total system failure.
Unfortunately, the amount of load shed under such schemes is typically quite conservative, often resulting in excessive or insufficient load shedding. This is because, typically, there is a complete lack of knowledge about the actual system operating conditions, as well as the types of disturbances and their locations within the system. Indeed, these load shedding schemes rely on pre-programmed load priority tables that are developed in advance of the power system's actual operation (i.e. during a “study phase”) and that are static (i.e. time invariant). The result is that the amount of load shed is not tailored to address actual power system conditions as they occur during operation.
Indeed, these conventional load shedding schemes have the inherent limitation that the contingencies, i.e. events resulting in power generation deficiency, on which these load priority tables are based are pre-programmed simulations which, at best, are simply educated guesses about what actual contingencies the power system may encounter and what the appropriate response would be. Hence, the associated load priority tables are only as good as the number of contingencies envisioned by the engineer who sets up the simulations. These tables will necessarily not include all possible contingencies, leaving the system vulnerable to contingencies that may cause system instability and/or failure. Moreover, these contingencies (and associated responses) while valid for the set of operating conditions for which they were created, are often inapplicable to the changing operating conditions of actual system operation. In short, the actual operating conditions of the power system are generally not those of the simulation.
It is therefore desirable to provide a proactive load shedding system that can predict the need for and the optimal type of responsive load shedding action to a contingency based on the actual operating conditions of a power system.